This invention relates to a process of making light olefins using a catalyst that has a low level of contamination. More specifically, this invention relates to the making of light olefins by employing a catalyst that retains a defined level of activity by reducing exposure of the catalyst to undesirable contaminants including crystalline silica, phosphates, alkaline metals and alkaline earth metals. These contaminants have been introduced into prior art reactors from refractory materials used within the reactors.
Olefins such as ethylene, propylene, the butenes, and the pentenes are useful in preparing a wide variety of end products including polyethylenes, polypropylenes, polyisobutylene and other polymers, alcohols, vinyl chloride monomer, acrylonitrile, methyl tertiary butyl ether and tertiary amyl methyl ether and other petrochemicals, and a variety of rubbers such as butyl rubber. Ethylene and propylene are two light olefins that are of particular value in producing such end products.
The olefins used in preparing olefin derivative products have traditionally been made by cracking hydrocarbon feedstocks or more recently by catalytically converting oxygenate feedstocks. Cracking of hydrocarbon feedstocks can be accomplished catalytically or non-catalytically. Non-catalytic cracking processes are described, for example, in Hallee et al., U.S. Pat. No. 3,407,789; DiNicolantonio et al., U.S. Pat. No. 4,499,055 and Gartside et al., U.S. Pat. No. 4,814,067. Catalytic cracking processes are described, for example, in Cormier, Jr. et al., U.S. Pat. No. 4,828,679; Rabo et al., U.S. Pat. No. 3,647,682; Rosinski et al., U.S. Pat. No. 3,758,403; Gartside et al., U.S. Pat. No. 4,814,067; Li et al., U.S. Pat. No. 4,980,053 and Yongqing et al., U.S. Pat. No. 5,326,465. Catalytic conversion of oxygenate feedstocks to produce olefins are described, for example in, Kaiser, U.S. Pat. No. 4,499,327; Barger, U.S. Pat. No. 5,095,163 and Hoelderich et al., U.S. Pat. No. 4,433,188.
Olefins which are typically used as feedstock in the preparation of the above described end products are supplied at a relatively high purity to the appropriate reaction unit.
There are a variety of catalytic processes that are carried out at relatively high temperatures in the presence of steam. In particular, solid acid catalysts for various catalytic processes, including, but not limited to, olefin cracking and methanol to olefin conversion are subject to high-temperature (typically, above 400° C.) exposure in the presence of steam, under process and/or regeneration conditions. In the case of regeneration conditions, in which the catalyst is subjected to oxidizing conditions, steam is formed upon combustion of coke. Catalyst degradation, which often occurs upon steaming, is a combination of several processes, such as zeolite dealumination (or, in general, a decrease in the number of acid sites), structural collapse, pores blockage, acid sites poisoning, as well as others. The rapid degradation of catalyst increases the amount of replacement catalyst that is required and can significantly increase operating costs.
In addition to the degradation of the catalyst performance through deposition of coke, it was unexpectedly found that under certain conditions the catalyst would tend to deactivate after a short period of use when catalyst was exposed to high temperature steaming in a quartz reactor. Analysis of the catalyst revealed that the catalyst was covered with a smooth layer of silica which isolated the active reaction sites from the process vapors. Further investigation revealed that the refractory materials used in the reactor were the source of this silica. It has been found that certain contaminants cause such catalyst degradation including commercial refractory materials used for reactor linings such as silicon, phosphorus, alkali and alkali earth metals. Under high temperatures these elements can become mobile and migrate from reactor lining to the catalyst which in turn leads to catalyst degradation and deactivation. The presence of steam can greatly promote the migration process. Previous to the present invention, there was an awareness that more extreme reactions, such as coal gasification which is carried out at temperatures of 982° C. (1800° F.) and pressures of 1034 kPa (150 psia) or more, experienced problems with lining materials leaching and damaging a catalyst. However, the reaction conditions for light olefin production are much lower. The present application describes a method for avoiding the undesired effects of acid sites poisoning and/or pores blockage by volatile silica and phosphorus species.